In the MRI apparatus, a test object is placed in homogeneous static magnetic field space, and imaging of the test object is performed by using nuclear magnetic resonance. An imaging region is limited to the static magnetic field space. In recent years, a method for imaging a total body has been developed, which moves a table (bed) on which the test object is placed, and now attention is particularly given to attempts of a total body screening by use of the MRI.
When measurement is performed as to a wide area such as the total body, it is desired to implement a receiver coil which is able to keep high sensitivity across the wide area, and simultaneously, it is also desired to shorten a time length necessary for the imaging, so as to make the measurement time to be within a range tolerable for the subject. As a technique for shortening the imaging time of the diagnostic MRI, a technique for developing image aliasing using sensitivity distributions from multiple RF coils (this technique being called as “parallel imaging”, and hereinafter, it will be referred to as “parallel imaging”) is coming into practical use (non patent document 1). In this method, a receiver coil made up of multiple sub-coils is used to perform simultaneous signal measurement, and the imaging time is shortened to a time length obtained by dividing original imaging time by the number of sub-coils.
In order to achieve the parallel imaging, it is necessary that electromagnetic coupling between the multiple sub-coils is sufficiently small. If there exists electromagnetic coupling between the sub-coils, noise interference may occur between the coils, and this may deteriorate an image S/N. Next, it is also necessary that the multiple sub-coils have to be arranged properly. If the arrangement of the sub-coils is not proper, the image S/N may be deteriorated partially. As one of the evaluation criteria to decide whether or not the arrangement of the sub-coils is appropriate, there is a standard referred to as Geometry factor (hereinafter, it will be abbreviated as “G factor”) (a calculation formula is described in non patent document 2). The G factor is a numerical value equal to or larger than 1.0 derived from a sensitivity distribution on an imaging plane as to each of the sub-coils, and the S/N at each position of an image is proportional to (1/(G factor)). Therefore, it is preferable that the G factor of the image at a part where the subject exists is as small as possible. At least, the value is desired to be smaller than 2.0, typically. As thus described, in order to design a receiver coil used for the parallel imaging, it is necessary to reduce the electromagnetic coupling between the multiple sub-coils used for the simultaneous measurement, and it is also necessary to find out a coil arrangement which allows the G factor to be a small value on all over the imaging plane. The parallel imaging has been developed mainly for a horizontal magnetic field apparatus having a high magnetic field, and various receiver coils are prepared for the horizontal magnetic field apparatus.
On the other hand, as for a vertical magnetic field open MRI apparatus, with its enhanced openness of magnet, the subject is directly accessible, and it is suitable for a usage as an interventional MRI. The direction of an RF magnetic field generated by the RF coil has to be orthogonal to the direction of the static magnetic field. Therefore, when the direction of the static magnetic field is changed from horizontal to vertical, it is necessary to change the receiver coil configuration as well. In the vertical magnetic field type MRI apparatus, because the direction of the static magnetic field is vertical, a subject is typically laid down in the horizontal direction when tested, and therefore a solenoid coil which is arranged around the outer periphery of the subject can be used. The solenoid coil which is arranged around the subject provides a strong sensitivity even in a deep portion of the subject, unlike a loop coil placed on the surface of the subject. Therefore, if the magnetic field strength is the same, the vertical magnetic field type MRI, in which the solenoid coil is usable, typically provides higher sensitivity in a deep portion of the subject, rather than the horizontal magnetic field type MRI.
By way of example, the patent document 1 and patent document 2 suggest the arrangement of the receiver coil which is compliant with the vertical magnetic field. The patent document 1 discloses a method for imaging with a high sensitivity and at a high speed with an application of parallel imaging, as to an area in proximity to a heart which is a deep portion of the subject, by using a combination of multiple solenoid coils arranged around the outer periphery of the subject and surface coils. The patent document 2 discloses that by using a solenoid and a saddle coil being orthogonal to each other, sensitivity in a deep portion of the subject is enhanced, and at least two sub-coils are arranged in opposed manner in each of the three directions of the subject, thereby forming a sensitivity profile of the sub-coils in the phase encoding direction of each of the three directions. By using the receiver coils with the arrangement as described above, it is possible to obtain high sensitivity even in a deep portion of the subject, and any phase encoding direction can be selected to achieve a high-speed imaging.
Non patent document 1    J. B. Ra, C. Y. Rim: “Fast Imaging Using Subencoding Data Sets from Multiple Detectors”, Magnetic Resonance in Medicine, vol. 30, pp. 142-145 (1993)
Non patent document 2    Klaas P. Pruessmann, Markus Weiger, Markus B. Scheidegger, and Peter Boesiger: “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine, vol. 42, pp. 952-962 (1999).
Patent document 1    Japanese Unexamined Patent Application Publication No. 2002-153440
Patent document 2    Japanese Unexamined Patent Application Publication No.